In the version of this article initially published, Fig. 7f purported to show an example of a multi-synapse spine. However, the structure in question included a mitochondrion and microtubules, meaning that it was actually a segment of dendritic shaft. A new image showing an example of a bona fide spine has been substituted. The legend has been changed to state that the image shows two boutons rather than three. The error has been corrected in the HTML and PDF versions of the article. The original and corrected figures are shown in the accompanying Author Correction.